(1) Field of the Invention
The present invention relates to electretization methods and apparatuses for dielectric films of condenser microphones formed using a micromachining technique for silicon.
(2) Description of Related Art
Electret condenser microphones (ECMs) are small acousto-electric transducers in which the need for a direct-current bias of a condenser is eliminated by detecting, as an electrical signal, a variation in the capacity of the condenser arising from an acoustic wave and utilizing an electret film possessing a semipermanent electric polarity.
An electret film (at least partially polarized dielectric film) of an ECM is made of, for example, an organic dielectric film, such as fluoroethylene propylene (FEP), and formed by injecting electric charges into this dielectric film and fixing the injected electric charges in the dielectric film. A potential difference is produced between both electrodes of a condenser, between which the dielectric film is interposed, by an electric field formed of the electric charges injected into the dielectric film. The process of injecting electric charges into a dielectric film and fixing the injected electric charges in the dielectric film is called “electretization”.
A dielectric film (electret film) is formed of a thin film made of FEP or any other material, and a metal, such as gold and nickel, is deposited on the outer surface of the thin film by evaporation or any other method.
Known methods for injecting charges into a dielectric film to form an electret film includes methods illustrated in FIGS. 6 and 7 (see, for example, Japanese Patent Laid-Open Publication No. 56-58220).
FIG. 6 is a cross-sectional view illustrating a principal part of a known apparatus for electretizing a dielectric film by causing corona discharge using a needle electrode.
In the apparatus illustrated in FIG. 6, a FEP thin film 4 is placed on a ground electrode (metal tray) 5, direct current corona discharge is caused by a needle electrode 6 connected to a high voltage power source 7, and resultant ions are injected into the FEP thin film 4 to fix the injected ions in the FEP thin film 4, thereby electretizing the FEP thin film 4.
FIG. 7 is a cross-sectional view illustrating a principal part of a known apparatus for electretizing a dielectric film by causing corona discharge using a wire electrode. In FIG. 7, the same components as those in FIG. 6 are denoted by the same reference numerals.
In the apparatus illustrated in FIG. 7, a FEP thin film 4 is placed on a ground electrode (metal tray) 5, direct current corona discharge is caused by a wire electrode 21 connected to a high voltage power source 7, and resultant ions are injected into the FEP thin film 4 to fix the injected ions in the FEP thin film 4, thereby electretizing the FEP thin film 4. Since in the apparatus illustrated in FIG. 7 the wire electrode 21 extends two-dimensionally, it has an advantage that ions can be extensively applied to the FEP thin film 4.
In view of the above, ECMs are generally fabricated in consideration of mass productivity in the following manner. A plurality of FEP thin films (dielectric films) are arranged on a metallic tray, and corona discharge is caused by the apparatus in FIG. 7, thereby electretizing a large number of FEP thin films at once. Since this method, however, may prevent ions from being uniformly applied to the FEP thin films, the FEP thin films may have different amounts of electret charges according to their locations on the tray. As a result, microphones have different sensitivities. Furthermore, in some cases, microphones have different sensitivities not only due to differences in the amount of electret charges among FEP thin films but also due to differences in parasitic capacitance and the capacities of field effect transistors (FETs) among microphones and other factors.
In the above-mentioned known example, a dielectric film itself to be electretized is taken out and then electretized. Such a known art can be said to be an art based on an ECM constructed by assembling mechanical parts.
On the other hand, in recent years, a technique has been suggested in which an ultrasmall condenser microphone is formed by micromachining a silicon substrate without assembling mechanical parts (see, for example, Japanese Patent Laid-Open Publication No. 11-88992, Japanese Patent Laid-Open Publication No. 2005-20411 and Published Japanese translation of a PCT application No. 2000-508860).
A condenser microphone of silicon fabricated by a technique for fabricating a so-called micro-electro-mechanical system (MEMS) element is referred to as a “silicon microphone (or silicon mic)”. A method for fabricating a silicon microphone using the technique for fabricating a MEMS element has received attention as a technique for fabricating an ECM for being incorporated into a cell phone terminal or the like whose reduction in size and thickness has been progressing (see, for example, Japanese Patent Laid-Open Publication No. 11-88992).
Since the silicon microphone is fabricated by processing a silicon substrate using a semiconductor process technique, this prevents a fabrication process for a silicon microphone from being able to include electretization that is not associated with a semiconductor process. In other words, it is impossible that a dielectric film alone is taken out and separately electretized.
In view of the above, a silicon microphone disclosed in, for example, Japanese Patent Laid-Open Publication No. 2005-20411 is a condenser microphone without an electret film.
However, it is not impossible that dielectric films of silicon microphones are electretized. For example, in a silicon microphone disclosed in Published Japanese translation of a PCT application No. 2000-508860, a dielectric film can be electretized.
More particularly, the silicon microphone disclosed in Published Japanese translation of a PCT application No. 2000-508860 includes a first silicon substrate (microphone film) including a dielectric film formed on the first silicon substrate and a second silicon substrate (microphone back plate) bonded to the first silicon substrate. The dielectric film is electretized in the last process step of a fabrication process for the first silicon substrate, and thereafter the first silicon substrate is bonded to the second silicon substrate.
According to a silicon microphone and a method for fabricating the same which are disclosed in International Patent Application No. PCT/JP2006/311248, in a silicon microphone formed of a single silicon substrate and having an unexposed dielectric film, the dielectric film can be electretized.
More particularly, an electretization method for the silicon microphone disclosed in International Patent Application No. PCT/JP2006/311248 is a method for electretizing dielectric films that are components of condenser microphones after mounting, on a substrate for packaging, chips for condenser microphone use which are formed by micromachining a silicon substrate. Each of condenser microphones is separately subjected to at least one corona discharge using a single needle electrode, thereby electretizing an associated dielectric film.
According to an electretization method disclosed in Japanese Patent Laid-Open Publication No. 2003-282360, when corona discharge is to be caused, a grid electrode is placed between an electrode used for corona discharge and an object to be electretized, thereby electretizing the object while controlling the motion of ions. In this way, the amount of electret charges can be controlled.